hmac
Section: OpenSSL (3)
Updated: 2017-05-25
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NAME
HMAC, HMAC_CTX_init, HMAC_Init, HMAC_Init_ex, HMAC_Update, HMAC_Final, HMAC_CTX_cleanup,
HMAC_cleanup - HMAC message authentication code
SYNOPSIS
#include <openssl/hmac.h>
unsigned char *HMAC(const EVP_MD *evp_md, const void *key,
int key_len, const unsigned char *d, int n,
unsigned char *md, unsigned int *md_len);
void HMAC_CTX_init(HMAC_CTX *ctx);
int HMAC_Init(HMAC_CTX *ctx, const void *key, int key_len,
const EVP_MD *md);
int HMAC_Init_ex(HMAC_CTX *ctx, const void *key, int key_len,
const EVP_MD *md, ENGINE *impl);
int HMAC_Update(HMAC_CTX *ctx, const unsigned char *data, int len);
int HMAC_Final(HMAC_CTX *ctx, unsigned char *md, unsigned int *len);
void HMAC_CTX_cleanup(HMAC_CTX *ctx);
void HMAC_cleanup(HMAC_CTX *ctx);
DESCRIPTION
HMAC is a
MAC (message authentication code), i.e. a keyed hash
function used for message authentication, which is based on a hash
function.
HMAC() computes the message authentication code of the n bytes at
d using the hash function evp_md and the key key which is
key_len bytes long.
It places the result in md (which must have space for the output of
the hash function, which is no more than EVP_MAX_MD_SIZE bytes).
If md is NULL, the digest is placed in a static array. The size of
the output is placed in md_len, unless it is NULL.
evp_md can be EVP_sha1(), EVP_ripemd160() etc.
HMAC_CTX_init() initialises a HMAC_CTX before first use. It must be
called.
HMAC_CTX_cleanup() erases the key and other data from the HMAC_CTX
and releases any associated resources. It must be called when an
HMAC_CTX is no longer required.
HMAC_cleanup() is an alias for HMAC_CTX_cleanup() included for back
compatibility with 0.9.6b, it is deprecated.
The following functions may be used if the message is not completely
stored in memory:
HMAC_Init() initializes a HMAC_CTX structure to use the hash
function evp_md and the key key which is key_len bytes
long. It is deprecated and only included for backward compatibility
with OpenSSL 0.9.6b.
HMAC_Init_ex() initializes or reuses a HMAC_CTX structure to use the hash
function evp_md and key key. If both are NULL (or evp_md is the same
as the previous digest used by ctx and key is NULL) the existing key is
reused. ctx must have been created with HMAC_CTX_new() before the first use
of an HMAC_CTX in this function. N.B. HMAC_Init() had this undocumented
behaviour in previous versions of OpenSSL - failure to switch to HMAC_Init_ex()
in programs that expect it will cause them to stop working.
NB: if HMAC_Init_ex() is called with key NULL and evp_md is not the
same as the previous digest used by ctx then an error is returned
because reuse of an existing key with a different digest is not supported.
HMAC_Update() can be called repeatedly with chunks of the message to
be authenticated (len bytes at data).
HMAC_Final() places the message authentication code in md, which
must have space for the hash function output.
RETURN VALUES
HMAC() returns a pointer to the message authentication code or
NULL if
an error occurred.
HMAC_Init_ex(), HMAC_Update() and HMAC_Final() return 1 for success or 0 if
an error occurred.
HMAC_CTX_init() and HMAC_CTX_cleanup() do not return values.
CONFORMING TO
RFC 2104
SEE ALSO
sha(3),
evp(3)
HISTORY
HMAC(),
HMAC_Init(),
HMAC_Update(),
HMAC_Final() and
HMAC_cleanup()
are available since SSLeay 0.9.0.
HMAC_CTX_init(), HMAC_Init_ex() and HMAC_CTX_cleanup() are available
since OpenSSL 0.9.7.
HMAC_Init_ex(), HMAC_Update() and HMAC_Final() did not return values in
versions of OpenSSL before 1.0.0.
Index
- NAME
-
- SYNOPSIS
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- DESCRIPTION
-
- RETURN VALUES
-
- CONFORMING TO
-
- SEE ALSO
-
- HISTORY
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